They answer the question from different directions. Reachability analysis examines code to judge whether your application can call the vulnerable part of a library. IAST watches the running app to see whether tested activity touches it. Reachability is broader. IAST is observed evidence for what ran.
What each one is
Reachability analysis is a static technique, generally built on a call graph. It asks whether there is a path from your code into the vulnerable function inside a dependency. If no path exists, the finding is likely low priority.
IAST puts sensors inside the running application. OWASP describes it as testing the application while an automated test, human tester, or any activity interacts with it. Sensors monitor behavior and alert on vulnerabilities as they appear.
Side by side
| Question | Reachability analysis | IAST |
|---|---|---|
| Needs the app running | No | Yes |
| Needs tests or traffic | No | Yes, coverage depends on them |
| Sees all code paths | Covers paths it can model | Only paths that execute |
| Handles reflection and dynamic loading | Can struggle | Sees what actually happens |
| Where it fits | Triage of dependency alerts | Finding flaws during testing |
| Setup | Usually point at code or build | Install an agent and drive traffic |
Treat the table as general characteristics. Specific products differ, so confirm with any vendor how a given tool handles dynamic code.
Where each one shines
Reachability is strong when you have hundreds of dependency alerts and need a defensible order for fixing them. It does not need a test environment, which makes it easy to apply across many repositories.
IAST is strong when you want evidence from execution, such as confirming that tainted input reaches a dangerous sink in a framework that static analysis models poorly.
Using them together
They are complementary. A reachable finding that IAST also observed is high confidence. An unreachable finding that never executed can wait. A finding that disagrees across the two is a good candidate for a short human review.
A worked example: one advisory, three services
An advisory appears for a library used in three services. Service A calls the vulnerable function directly in a request handler. Service B includes the library but only uses unrelated utilities. Service C calls the function, but only from an internal job that runs on trusted input.
Reachability analysis would flag A as reachable, B as not reachable, and C as reachable. Priority: fix A first. IAST, if running on A during tests that hit the handler, might also observe the call, adding confidence. For C, the team applies judgment about exposure and schedules it after A. B waits for the next routine upgrade. The three decisions come from combining evidence, not from a single tool's verdict.
What the combination buys you
- A shorter list: unreachable items stay off the urgent queue.
- Better confidence: items seen both ways are strong candidates.
- Clear escalations: disagreements go to a short human review.
- Defensible records: you can show why an item was deferred.
Practical sequencing
Start with reachability across all repositories, since it needs no test environment. Layer IAST onto the few services where runtime evidence will change decisions. Reassess when you add services or upgrade frameworks, because both techniques depend on how well they model your stack.
Questions to ask vendors about either technique
For reachability: which languages are modeled, how are dynamic calls handled, and how are results explained to a developer? For IAST: which frameworks are supported, how is coverage shown, and what is the overhead? For both: how do results flow into tickets and gates, and how can a verdict be overridden with an audit trail? Asking the same questions of each vendor makes comparison fair, and exposes where marketing claims go further than the product.
Summary decision guide
If you have many dependency alerts and limited time, begin with reachability. If you have strong integration tests on critical services, add IAST there for runtime evidence. If your stack uses heavy reflection, weigh runtime evidence more. If a finding is flagged by both, fix it first. If neither tool sees it, review whether the tools support your stack. Revisit the setup when frameworks change.
Reporting the combined view
In reports, show three groups: reachable and observed, reachable but not observed, and neither. The first group is the urgent queue, the second is next, and the third is routine. A simple three bucket view is easy for leaders to read and makes the reasoning behind priorities visible, which helps when teams ask why one item was fixed before another.
Try it yourself
Click through it at your own pace, or open it full screen.
See how filtering by reachability changes the size of the list a team has to work through.
A decision guide
- Many dependency alerts, few people: start with reachability.
- A critical service with strong tests: consider adding IAST there.
- Heavy use of reflection or plugins: expect reachability to be less certain, and weigh runtime evidence more.
Where each one falls short
Reachability analysis can be fooled by code it cannot model, such as reflection, plugin systems, or dynamic loading, so a no path result deserves a second look in those frameworks. IAST can only speak for the activity it saw, so a library function that never ran during tests tells you nothing either way. Knowing these gaps lets you decide when to trust a verdict and when to ask for another opinion.
Quick check
Does reachability analysis replace IAST?
No. One judges possible calls, the other observes real execution during testing. They overlap on dependencies and differ on everything else.
Can reachability prove a flaw is safe?
It can show no modeled path exists. Dynamic features can hide paths, so treat the result as strong evidence, not a guarantee.
Can I use reachability alone?
Many teams do, especially at first. Add runtime evidence where tests are strong and decisions are hard.
Safeguard offers reachability analysis for dependency findings, plus SBOM and dependency scanning, with a free scan at gold.safeguard.sh/scan. For the testing side, see what IAST is.
Sources
Last reviewed: 19 October 2026.
